JP7762015B2 - Ultrasonic Flow Sensor - Google Patents

Description

The present invention relates to an ultrasonic flow sensor that measures the flow rate of a fluid flowing through a pipe.

Ultrasonic flow sensors are known that are attached to pipes to measure the flow rate of fluid flowing through the pipes. For example, Patent Document 1 describes an ultrasonic flow switch that includes first and second ultrasonic elements. The first ultrasonic element transmits ultrasonic waves into the pipe, and the second ultrasonic element receives the ultrasonic waves reflected by the inner wall of the pipe. The second ultrasonic element then transmits ultrasonic waves into the pipe, and the first ultrasonic element receives the ultrasonic waves reflected by the inner wall of the pipe. The flow rate of the fluid flowing through the pipe is measured based on the difference in propagation time of the ultrasonic waves between the ultrasonic elements.

Patent No. 6453156

Ultrasonic flow sensors are used, for example, to measure the flow rate of coolant liquid that cools the machining point of a machine tool. However, coolant liquid contains a large amount of tiny bubbles (microbubbles). In this case, the ultrasonic waves transmitted by each ultrasonic element are attenuated in the coolant liquid by the microbubbles. As a result, the ultrasonic flow switch described in Patent Document 1 may not be able to properly measure the flow rate of coolant liquid.

The object of the present invention is to provide an ultrasonic flow sensor capable of measuring the flow rate of a fluid containing microbubbles.

(1) An ultrasonic flow sensor according to the present invention is an ultrasonic flow sensor for measuring the flow rate of a fluid containing microbubbles flowing through a pipe, the ultrasonic flow sensor comprising: a first ultrasonic element that transmits to the fluid a pulsed first ultrasonic signal, which is an ultrasonic wave having a predetermined frequency included in the frequency band of 2 MHz to 5 MHz, which is higher than the absorption band of the microbubbles; a second ultrasonic element that receives a second ultrasonic signal having the predetermined frequency transmitted from the first ultrasonic element to the fluid and transmits to the fluid a third ultrasonic signal received by the first ultrasonic element; and a calculation unit that calculates, as a first method of flow rate calculation, a first flow rate of the fluid based on the difference between the predetermined frequency and the frequency of the ultrasonic signal reflected by the microbubbles , and calculates, as a second method of flow rate calculation, a second flow rate of the fluid based on the difference in propagation time between the second ultrasonic signal and the third ultrasonic signal between the first ultrasonic element and the second ultrasonic element, and that outputs a calculated flow rate based on at least one of the first flow rate and the second flow rate, while accommodating microbubbles having a density whose flow rate can be calculated by both the first method and the second method, and also accommodating microbubbles having a density whose flow rate can be calculated by one of the first method and the second method but that exceeds the density range whose flow rate can be calculated by both the first method and the second method .

In this ultrasonic flow sensor, the pulsed first ultrasonic signal transmitted by the first ultrasonic element has a frequency higher than the absorption band of microbubbles, reducing the attenuation of the ultrasonic signal due to microbubbles. This makes it possible to measure the flow rate of a fluid even if the fluid contains microbubbles.

Furthermore, the predetermined frequency of the ultrasonic signal is 2 MHz or more and 5 MHz or less, in which case the attenuation of the ultrasonic signal due to microbubbles can be more reliably reduced.
The ultrasonic flow sensor further includes a second ultrasonic element that receives a second ultrasonic signal transmitted from the first ultrasonic element to the fluid and transmits a third ultrasonic signal received by the first ultrasonic element to the fluid, and the calculation unit calculates a second flow rate of the fluid based on a propagation time difference between the second ultrasonic signal and the third ultrasonic signal between the first ultrasonic element and the second ultrasonic element, and outputs a calculated flow rate based on at least one of the first flow rate and the second flow rate. With this configuration, the flow rate of the fluid can be accurately measured even when the density of microbubbles contained in the fluid is low.

( 2 ) The calculation unit may calculate the fluid velocity at a first depth in the pipe based on the difference between a predetermined frequency and the frequency of an ultrasonic signal reflected by a microbubble at a first time point a predetermined time after the ultrasonic signal is transmitted by the first ultrasonic element, calculate the fluid velocity at a second depth in the pipe based on the difference between the predetermined frequency and the frequency of an ultrasonic signal reflected by a microbubble at a second time point different from the first time point after the ultrasonic signal is transmitted by the first ultrasonic element, and calculate the first flow rate based on a representative value of the fluid velocity distribution determined based on the fluid velocity at the first depth and the fluid velocity at the second depth. In this case, the fluid flow rate can be measured more accurately.

( 3 ) The first ultrasonic element may be a composite element. In this case, the reverberation time of the ultrasonic waves transmitted by the first ultrasonic element is short. This allows for more accurate measurement of the fluid flow rate.

( 4 ) The ultrasonic flow sensor may further include a determination unit that determines a combination ratio of the first flow rate and the second flow rate based on the stability of the flow rate calculation, and the calculation unit may calculate a third flow rate of the fluid by combining the first flow rate and the second flow rate at the combination ratio determined by the determination unit. In this case, it becomes easy to accurately measure the flow rate of the fluid regardless of the density of microbubbles contained in the fluid.

( 5 ) The ultrasonic flow sensor may further include a detection unit that detects the density of microbubbles contained in the fluid, and the calculation unit may correct the calculated first flow rate based on the density of microbubbles detected by the detection unit. With this configuration, the flow rate of the fluid can be accurately measured even when the density of microbubbles contained in the fluid is relatively high.

( 6 ) The ultrasonic flow sensor may further include a determination unit that determines whether a cycle skip occurred when calculating the first flow rate, and the calculation unit may correct the calculated first flow rate when the determination unit determines that a cycle skip occurred. With this configuration, the flow rate of the fluid can be accurately measured even if a cycle skip occurred when calculating the first flow rate.

( 7 ) The ultrasonic flow sensor may further include a sensor head having a first ultrasonic element and a sensor head housing that houses the first ultrasonic element. In this case, the first ultrasonic element can be easily attached to the pipe by attaching the sensor head housing to the pipe. This makes it possible to easily calculate the flow rate of the fluid flowing through the pipe to which the sensor head is attached.

( 8 ) The ultrasonic flow sensor further includes a display that communicates with the sensor head, the sensor head having a sensor head connector, and the display having a display connector that connects to the sensor head connector, a display unit that displays display information based on the calculated flow rate transmitted from the sensor head, a display control unit that controls the display unit, and a display housing that accommodates the display control unit, and the sensor head connector and the display connector may be connected by a first connection method in which the display housing is directly connected by being attached to the sensor head housing, or a second connection method in which they are connected via a cable.

In this case, with the first connection method, the user can easily recognize the displayed information by visually checking the display attached to the sensor head. Furthermore, with the second connection method, the user can easily recognize the displayed information by visually checking the display attached to any position.

( 9 ) The sensor head may include a sensor head indicator light, and the display control unit may generate a switching signal indicating whether the flow rate calculated by the calculation unit is equal to or greater than a predetermined threshold value or whether it is less than the predetermined threshold value. The sensor head indicator light may display the result of comparing the calculated flow rate with the predetermined threshold value. In this case, the ultrasonic flow sensor can be operated as a flow switch. Furthermore, the user can easily recognize the result of comparing the calculated flow rate with the predetermined threshold value.

The present invention makes it possible to measure the flow rate of a fluid containing microbubbles.

1 is a side view of an ultrasonic flow sensor according to an embodiment of the present invention. FIG. 2 is a schematic cross-sectional view showing the configuration of the sensor head of FIG. 1 . 10A and 10B are diagrams for explaining the operation of a sensor head in a transit time method. 10A and 10B are diagrams for explaining the operation of a sensor head in a pulse Doppler system. 10A and 10B are diagrams for explaining more specific operation of a sensor head in a pulse Doppler system. FIG. 10 is a diagram showing Doppler signals detected over time. FIG. 10 is a diagram showing the flow rate of a fluid calculated by a transit time method and a pulse Doppler method. FIG. 10 is a diagram illustrating a typical example of frequency characteristics of a structural filter. FIG. 1 is a diagram showing the resonant frequency characteristics of microbubbles contained in a fluid. FIG. 10 is a diagram showing a Doppler signal detected when the density of microbubbles is high. FIG. 3 is a block diagram showing the configuration of a control unit 13 in FIG. 2 . 12 is a flowchart showing an example of an algorithm for a flow rate calculation process executed by the control unit in FIG. 11 in a hybrid mode. 12 is a flowchart showing an example of an algorithm for a flow rate calculation process executed by the control unit in FIG. 11 in a hybrid mode.

(1) Schematic Configuration of Ultrasonic Flow Sensor An ultrasonic flow sensor according to an embodiment of the present invention will now be described with reference to the drawings. FIG. 1 is a side view of an ultrasonic flow sensor according to an embodiment of the present invention. As shown in FIG. 1, the ultrasonic flow sensor 100 is composed of a sensor head 10, a clamp unit 20, and a display 30. The sensor head 10 is attached to a pipe P and calculates the flow rate of a fluid flowing through the pipe P. The maximum inner diameter (diameter) of the pipe P is an inner diameter corresponding to a nominal pipe diameter of "50A" specified in the JIS standard, for example. For example, the inner diameter of a steel pipe with a nominal pipe diameter of "50A" is 52.9 mm. In the following description, an example will be described in which the sensor head 10 is attached to the top surface of a horizontally extending pipe P.

The sensor head 10 includes a connector 12. The connector 12 is provided on the top surface of the sensor head 10.

The clamp unit 20 includes an upper clamp member 21 and a lower clamp member 22. The upper clamp member 21 and the lower clamp member 22 are positioned to sandwich the piping P and are connected to each other by a plurality of clamp fixing screws 23. This attaches the clamp unit 20 to the outer surface of the piping P. As indicated by the dashed-dotted arrows in Figure 1, two sensor fixing screws 101 are threaded into the upper surface of the upper clamp member 21 through the sensor head 10. This holds the sensor head 10 in place by the clamp unit 20 with its underside in contact with the piping P.

The display 30 includes a housing 31, a connector 32, a control unit 33, a memory element 34, an operation unit 35, a display unit 36, an indicator light 37, connection ports 38 and 39, and a power supply circuit 40. The housing 31 has a roughly rectangular parallelepiped shape and can be attached to the top surface of the sensor head 10 as indicated by the dotted arrow in Figure 1. The control unit 33, the memory element 34, and the power supply circuit 40 are provided inside the housing 31.

The connector 32 is provided on the underside of the housing 31. Connecting the connector 12 and the connector 32 enables communication between the display 30 and the sensor head 10. In this embodiment, the connector 12 and the connector 32 are connected in two ways: directly and indirectly. When the connector 12 and the connector 32 are connected directly, the housing 31 is attached to the top surface of the sensor head 10, thereby connecting the connector 12 and the connector 32. When the connector 12 and the connector 32 are connected via a cable (not shown), the housing 31 may be attached to the sensor head 10 or may be detached from the sensor head 10. In other words, because the connector 12 and the connector 32 are connected in two ways, the connector 32 of the display 30 and the connector 12 of the sensor head 10 are connected while the display 30 is detachable from the sensor head 10.

The control unit 33 includes, for example, a CPU (central processing unit) and a memory unit. The control unit 33 controls the operation of the memory element 34, the display unit 36, and the indicator light 37. The control unit 33 also compares the flow rate calculated by the sensor head 10 with a predetermined threshold and generates a switching signal based on the comparison result. Therefore, the generated switching signal is a binary signal indicating whether the flow rate value calculated by the sensor head 10 is in one of two states: a state where the flow rate value is equal to or greater than the predetermined threshold, or a state where the flow rate value is less than the predetermined threshold. This switching signal is used by an external device capable of controlling other devices to switch the other devices between an on state and an off state depending on the flow rate of the fluid flowing through the pipe P. In this way, when the other devices are controlled depending on the flow rate of the fluid flowing through the pipe P, the ultrasonic flow sensor 100 can be said to function as a flow switch that changes the operating state of the other devices depending on whether the fluid flowing through the pipe P is at a flow rate equal to or greater than the threshold.

In this embodiment, the switching signal is generated by comparing the flow rate value calculated by the sensor head 10 with a predetermined threshold value, but any control may be used as long as the switching signal reflects whether the flow rate of the fluid flowing through the pipe P is above a certain amount or below a certain amount. As will be described later, in this example, the flow rate value calculated by the sensor head 10 is calculated based on the flow velocity of the fluid calculated by the sensor head 10 and the cross-sectional area of the pipe P. Therefore, for example, the calculated flow velocity may be compared with a threshold value related to the flow velocity to generate the switching signal.

The memory element 34 includes, for example, a ring buffer. The memory element 34 sequentially stores log data at predetermined time intervals, in which time is associated with the flow rate calculated by the sensor head 10. The log data may include maximum flow rate, minimum flow rate, integrated flow rate, or the level of the switching signal. When log data is stored in the entire memory area of the memory element 34, the oldest stored log data is overwritten with the latest log data. Therefore, log data stored in the memory element 34 is retained for a certain period of time until it is overwritten by the latest log data.

The operation unit 35 is provided on the top surface of the housing 31 so that it can accept input operations by the user. The user can input various parameters by operating the operation unit 35. Parameters input from the operation unit 35 include parameters related to the initial settings for calculating the flow rate, frequently used parameters, and infrequently used parameters. Parameters related to the initial settings include the material of the pipe P or the outer diameter of the pipe P according to the standard. For user convenience, it is preferable that the outer diameter of the pipe P input as the parameter related to the initial settings be the nominal diameter defined by the standard. Frequently used parameters include response time, display resolution, hysteresis, zero cutoff flow rate, fluid flow direction, or the period for detecting microbubble density (described below). These frequently used parameters are related to the switching signal. For this reason, when the user adjusts the output based on the actual output results from the ultrasonic flow sensor 100, they are set to arbitrary values. Therefore, it is expected that these parameters will be used frequently. Infrequently used parameters include the flow rate calculation mode, the outer diameter of the pipe P, the thickness of the pipe P, the sound velocity in the pipe P, or the dynamic viscosity of the fluid. These parameters affect the flow rate value calculated by the sensor head 10. For this reason, they are changed to any value when the user adjusts the process leading up to the output of the ultrasonic flow sensor 100, rather than the output itself. Therefore, they are expected to be used infrequently. Note that the parameter related to the dimensions of the pipe P, which is included in the infrequently used parameters, is a parameter whose value is changed when the user fine-tunes the value corresponding to the nominal diameter. Some or all of the above parameters may be input in a selective manner. The parameters input from the operation unit 35 are provided to the sensor head 10. The user can also input a desired threshold value for the flow rate by operating the operation unit 35.

The display unit 36 is provided on the top surface of the housing unit 31. The display unit 36 displays the fluid flow rate calculated by the sensor head 10. In other words, the display 30 equipped with the display unit 36 that displays the flow rate is detachable from the sensor head 10. Therefore, by placing the display 30 in a suitable position, the user can visually check the flow rate in the portion of the piping P where the sensor head 10 is installed.

The indicator light 37 includes, for example, multiple light-emitting diodes that emit light in different colors, and is provided on the top surface of the housing 31. The indicator light 37 lights up or flashes in a manner that allows the level of the switching signal generated by the control unit 33 to be distinguished. That is, the indicator light 37 lights up or flashes in a manner that allows the indication that the flow rate of the fluid flowing through the piping P is equal to or greater than a certain amount, and that the flow rate of the fluid flowing through the piping P is less than a certain amount. Like the display unit 36, the indicator light 37 is provided on the display 30, so the user can visually check the flow rate status of the portion of the piping P where the sensor head 10 is provided, from a convenient position.

The connection port 38 includes, for example, a USB (Universal Serial Bus) port and is provided on the side of the housing 31. When the connection port 38 is connected to an external information processing device via a cable (not shown), the log data stored in the memory element 34 is output to the information processing device. The connection port 39 includes, for example, an M12 port and is provided on the end face of the housing 31. When the connection port 39 is connected to an external device such as a personal computer or programmable logic controller via a cable (not shown), the switching signal generated by the control unit 33 is output to the external device.

The power supply circuit 40 converts the voltage supplied by an external commercial power source into a voltage suitable for the ultrasonic flow sensor 100, and supplies the converted voltage to the control unit 33, memory element 34, display unit 36, and indicator light 37. The power supply circuit 40 also supplies the converted voltage to each part of the sensor head 10 through the connector 32.

Figure 2 is a schematic cross-sectional view showing the configuration of the sensor head 10 in Figure 1. As shown in Figure 2, the sensor head 10 includes a housing 11, a connector 12, a control unit 13, two wedge members 14, two ultrasonic elements 15, an acoustic couplant 16, an ultrasonic shielding plate 17, and an indicator light 18.

The housing 11 is roughly rectangular and open at the bottom. The connector 12 is exposed on the top surface of the housing 11. The wedge material 14 is exposed at the opening at the bottom of the housing 11. The housing 11 houses the control unit 13 and the ultrasonic element 15.

The connector 12 is provided on the top surface of the housing 11. As described above, the connector 12 is positioned so that it can be connected to the connector 32 when the housing 31 of the display 30 in Figure 1 is attached to the housing 11.

The control unit 13 includes, for example, a CPU and a memory unit, and is provided inside the housing unit 11. The control unit 13 controls the operation of the two ultrasonic elements 15 and the indicator light 18. The control unit 13 also calculates the flow rate of the fluid flowing through the pipe P based on the measurement results obtained by the operation of the ultrasonic elements 15, parameters provided by the display 30, and predetermined parameters. The method for calculating the flow rate will be described in detail below.

Each wedge member 14 is formed from a non-metallic material that has high rigidity and high acoustic transparency. It is also preferable that each wedge member 14 be formed from a material that has high environmental resistance. In this example, each wedge member 14 is formed from PPS (polyphenylene sulfide) resin and PEEK (polyether ether ketone) resin, but it may also be formed from ULTEM (registered trademark) resin. Each wedge member 14 has an element coupling surface 14a facing diagonally upward and a piping coupling surface 14b facing downward.

In the following description, when distinguishing between the two wedge materials 14, one wedge material 14 will be referred to as wedge material 14A and the other wedge material 14 will be referred to as wedge material 14B. Wedge materials 14A and 14B are attached to the opening at the bottom of the housing unit 11, arranged in the longitudinal direction of the housing unit 11 with their element coupling surfaces 14a facing diagonally upward and outward. This creates a space inside the housing unit 11 that liquids such as water and oil cannot penetrate.

Each ultrasonic element 15 is configured to be able to selectively operate in a transmit mode in which ultrasonic waves are transmitted, and a receive mode in which ultrasonic waves are received. Each ultrasonic element 15 may be configured as a composite element. In this case, the reverberation time of the transmitted ultrasonic waves is short, thereby reducing noise when the same ultrasonic element 15 operates in a transmit mode and then a receive mode. In the pulse Doppler method described below, it is preferable that the ultrasonic element 15 that transmits ultrasonic waves receive the ultrasonic signal reflected by the reflector, so that the ultrasonic signal reflected by the reflector is received at a location where the strength of the ultrasonic signal is high. Therefore, in the pulse Doppler method, it is preferable that the ultrasonic element 15 be a composite element to improve the received strength of the ultrasonic signal. Note that the transit time differential method also has a certain effect, as it reduces noise when operating in a transmit mode and then a receive mode.

In the following description, when distinguishing between the two ultrasonic elements 15, one ultrasonic element 15 will be referred to as ultrasonic element 15A and the other ultrasonic element 15 will be referred to as ultrasonic element 15B. Ultrasonic elements 15A and 15B are joined to the element joining surfaces 14a of wedge materials 14A and 14B, respectively. As a result, ultrasonic elements 15A and 15B are housed in the housing 11 at a predetermined angle relative to the piping P.

The acoustic couplant 16 has a solid shape and is formed from a soft elastic material such as polymer rubber or a gel-like substance. The acoustic couplant 16 is provided at the bottom of the housing 11 so as to contact the pipe coupling surface 14b of each wedge 14. The bottom surface of the acoustic couplant 16 protrudes slightly from the bottom surface of the housing 11. The acoustic couplant 16 matches the acoustic impedance of each wedge 14 and the pipe P by contacting its bottom surface with the pipe P. Therefore, it is preferable that the acoustic couplant 16 have an acoustic impedance value between the acoustic impedance value of each wedge 14 and the acoustic impedance value of the pipe P.

The ultrasonic shielding plate 17 is made of, for example, foam rubber and has a flat plate shape. When upright, the ultrasonic shielding plate 17 is placed between the wedges 14A and 14B so that it penetrates the acoustic couplant 16. In this case, ultrasonic components that do not pass through the piping P are prevented from being transmitted directly between the wedges 14A and 14B.

The indicator light 18 includes, for example, multiple light-emitting diodes, and lights up or flashes in a manner that allows the level of the switching signal to be identified, similar to the indicator light 37 of the display 30. Therefore, when the display 30 is directly connected to the sensor head 10, the user can easily identify the level of the switching signal by visually checking the indicator light 37 of the display 30. On the other hand, the display 30 can be separated from the sensor head 10 by connecting the two with a cable. In this state, the user can install the display 30 on any installation surface. Even when the display 30 is separated from the sensor head 10, the user can easily identify the level of the switching signal by visually checking the indicator light 18 of the sensor head 10.

As described above, the sensor head 10 is fixed to the clamp unit 20 in Figure 1 with two sensor fixing screws 101. This allows the sensor head 10 to be attached to the pipe P with the acoustic couplant 16 pressed against the pipe P. In this state, the sensor head 10 operates. The operation of the sensor head 10 is described below.

(2) Operation of the Sensor Head By operating the operation unit 35 in Fig. 1, the user can select one of the transit time difference mode, pulse Doppler mode, and hybrid mode as the flow rate calculation mode of the sensor head 10. In the transit time difference mode, the sensor head 10 operates using the transit time difference method to calculate the flow rate. In the pulse Doppler mode, the sensor head 10 operates using the pulse Doppler method to calculate the flow rate. In the hybrid mode, the sensor head 10 operates using both the transit time difference method and the pulse Doppler method to calculate the flow rate.

Figure 3 is a diagram illustrating the operation of the sensor head 10 using the transit time differential method. In the transit time differential method, ultrasonic element 15A first transmits ultrasonic waves, and ultrasonic element 15B receives them. The ultrasonic waves transmitted diagonally downward by ultrasonic element 15A are incident on wedge material 14A as shown by arrow A1. The ultrasonic waves incident on wedge material 14A pass through acoustic couplant 16 and are incident on the fluid in pipe P as shown by arrow A2. The ultrasonic waves that pass through the fluid are reflected by the inner wall of pipe P and propagate through the fluid in the direction of arrow A3. The ultrasonic waves propagating in the direction of arrow A3 pass through acoustic couplant 16 and are incident on wedge material 14B as shown by arrow A4. The ultrasonic waves incident on wedge material 14B are received by ultrasonic element 15B. The control unit 13 measures the propagation time AB, which is the time it takes for the ultrasonic waves transmitted by ultrasonic element 15A to be received by ultrasonic element 15B.

After ultrasonic element 15B receives the ultrasonic waves transmitted from ultrasonic element 15A, ultrasonic element 15B transmits ultrasonic waves, and ultrasonic element 15A receives the ultrasonic waves transmitted from ultrasonic element 15B. The ultrasonic waves transmitted diagonally downward by ultrasonic element 15B are incident on wedge material 14B as shown by arrow B1. The ultrasonic waves incident on wedge material 14B pass through acoustic couplant 16 and are incident on the fluid in pipe P as shown by arrow B2. The ultrasonic waves that pass through the fluid are reflected by the inner wall of pipe P and propagate within the fluid in the direction of arrow B3. The ultrasonic waves propagating in the direction of arrow B3 pass through acoustic couplant 16 and are incident on wedge material 14A as shown by arrow B4. The ultrasonic waves incident on wedge material 14A are received by ultrasonic element 15A. The control unit 13 measures the propagation time BA, which is the time it takes for the ultrasonic waves transmitted by ultrasonic element 15B to be received by ultrasonic element 15A.

After measuring the propagation times AB and BA, the control unit 13 measures the propagation time difference Δt. The propagation time difference Δt is the difference between the propagation times AB and BA. Based on the propagation time difference Δt, the control unit 13 calculates the flow rate V of the fluid flowing through the pipe P using the following formula (1):

Here, C' is the ultrasonic velocity in the wedge material 14, and θ' is the angle of incidence of the ultrasonic wave on the wedge material 14. C is the ultrasonic velocity in the fluid, and θ is the angle of incidence of the ultrasonic wave on the fluid. d is the inner diameter of the pipe P, and λ is the pipe friction coefficient (Blasius coefficient). The velocity C', angle of incidence θ', angle of incidence θ, and Blasius coefficient are known and are stored as predetermined parameters in the memory unit of the control unit 13.

In calculating the flow rate by the transit time difference method in transit time difference mode, the user inputs the velocity C and the inner diameter d of the pipe P. Note that d ² π/4 in equation (1) or equation (2) described below is the cross-sectional area of the pipe P, and the value obtained by dividing the flow rate V by the cross-sectional area is the flow velocity of the fluid flowing through the pipe P.

As such, the transit time differential method requires two ultrasonic elements: one that transmits an ultrasonic signal that propagates along the fluid flow direction, and another that transmits an ultrasonic signal that propagates against the fluid flow direction. Each of these ultrasonic elements is positioned so that it can receive the ultrasonic signal transmitted from the other ultrasonic element. In this example, ultrasonic elements 15A and 15B are both positioned above the piping P, so ultrasonic elements 15 can be positioned by working from one side of the piping P. In particular, in this example, both ultrasonic elements 15A and 15B are housed in the housing 11, making installation of the sensor head 10 easy.

In this embodiment, the velocity C is input by the user, but the speed of sound in the pipe P may also be input. Because the value of the speed of sound in the pipe P, determined from the material of the pipe P, is unlikely to deviate from the actual value, the user can easily input a highly accurate value by referring to a table correlating the material of the pipe P with the speed of sound. The speed of sound in the pipe P is used to calculate the propagation time in the fluid from the propagation time from one side of the ultrasonic element 15 to the other. However, because the proportion of the pipe P in the propagation path from one side of the ultrasonic element 15 to the other is minimal, the impact of errors in the propagation speed of ultrasound in the pipe P on the calculated flow rate V is small. Therefore, the required accuracy of the value input by the user is low, allowing the user to input the value using a simple method. In an example where the speed of sound in the pipe P is input, the velocity in the fluid of the ultrasonic signal transmitted using the transit time differential method is calculated via the propagation time of the ultrasonic signal in the fluid. Because this velocity is a value approximating the velocity C, a flow rate V approximating the flow rate V when the velocity C is input is calculated. In this way, by appropriately changing the input values, the burden on the user of inputting values can be reduced. Note that the user can select the pipe material and nominal diameter defined by the standard, and the control unit 13 can calculate the sound speed in the pipe P based on this information.

Figure 4 is a diagram illustrating the operation of the sensor head 10 in the pulse Doppler system. In the pulse Doppler system, the operating mode of the ultrasonic element 15B alternates between transmission mode and reception mode. In transmission mode, the ultrasonic element 15B transmits several pulses of ultrasonic signals. The ultrasonic waves transmitted diagonally downward by the ultrasonic element 15B are incident on the wedge material 14B as shown by arrow C1. The ultrasonic waves incident on the wedge material 14B pass through the acoustic couplant 16 and are incident on the fluid in the pipe P as shown by arrow C2.

The fluid flowing through the pipe P contains microbubbles that act as reflectors of ultrasonic waves. Microbubbles are tiny air bubbles with diameters of, for example, 10 μm to 50 μm. Microbubbles are also likely to be generated in coolant fluids such as water-soluble cutting oils that cool the machining points of machine tools. Coolant fluids contain surfactants, and any coolant that comes into contact with air is circulated and reused. This means that microbubbles are likely to be generated in large quantities in the coolant fluid flowing through the pipe P, which forms part of the circulation system.

A portion of the ultrasound reflected by the microbubbles passes through the acoustic couplant 16 as shown by arrow C3 and is incident on the wedge material 14B as shown by arrow C4. The ultrasound incident on the wedge material 14B is received by the ultrasonic element 15B in receive mode. Thus, in the pulse Doppler method, the time it takes for the ultrasound transmitted from the ultrasonic element 15B to be reflected by the microbubbles and reach the ultrasonic element 15B is short, and is determined by the position of the microbubbles, so it is preferable that the ultrasonic element 15B be a composite element with a short reverberation time.

The control unit 13 measures the Doppler frequency Δf. The Doppler frequency Δf is the difference between the frequency of the ultrasound transmitted by the ultrasonic element 15B and the frequency of the ultrasound received by the ultrasonic element 15B, and is proportional to the flow velocity of the microbubbles flowing through the pipe P, i.e., the flow velocity of the fluid. Based on the measured Doppler frequency Δf, the control unit 13 calculates the flow rate V of the fluid flowing through the pipe P using the following equation (2). Here, f is the frequency of the ultrasound transmitted by the ultrasonic element 15B. The frequency f is known and is stored as a predetermined parameter in the memory unit of the control unit 13. Details of the frequency f will be described later.

As such, the pulse Doppler method simply requires that a pulsed ultrasonic signal be transmitted into a fluid and that the frequency of the ultrasonic signal be measured when the ultrasonic signal is reflected by a reflector contained in the fluid. In this example, a single ultrasonic element 15B transmits a pulsed ultrasonic signal and measures the frequency of the ultrasonic signal reflected by microbubbles, minimizing the number of ultrasonic elements required for the pulse Doppler method and contributing to a certain degree to the miniaturization of the sensor head 10. In particular, in this example, the ultrasonic element 15B is composed of a composite element with a short reverberation time, so that when measuring the frequency of the ultrasonic signal reflected by microbubbles, the reverberation that occurs when the ultrasonic signal is transmitted is less likely to have an effect.

The sensor head 10, which calculates the flow rate using the pulse Doppler method, may also include an ultrasonic element that measures the frequency of ultrasonic waves reflected by a reflector, separate from the ultrasonic element that transmits the ultrasonic signal. In this case, because the ultrasonic element that measures the frequency is separate from the ultrasonic element that transmits the ultrasonic signal, detection accuracy is less likely to be reduced by reverberation.

Figure 5 is a diagram for explaining the operation of the sensor head 10 in more detail using the pulse Doppler method. As shown in Figure 5, the fluid does not flow at a uniform speed within the flow path of the pipe P, but rather flows with a predetermined velocity distribution. The flow velocity of the fluid flowing near the center of the flow path of the pipe P is greater than the flow velocity of the fluid flowing near the inner wall of the pipe P.

Here, in the pulse Doppler method, the time it takes for the ultrasonic waves transmitted by the ultrasonic element 15B to reach the microbubbles and the time it takes for the ultrasonic waves reflected by the microbubbles to reach the ultrasonic element 15B differ depending on the depth to which the microbubbles flow. In this example, the depth to which the microbubbles flow refers to the position of the microbubbles in the radial direction of the pipe P. Therefore, the control unit 13 measures the Doppler frequency for each time period from when the ultrasonic waves are transmitted until the ultrasonic waves reflected by the microbubbles are received.

In this embodiment, connector 12 is located closer to ultrasonic element 15A than to ultrasonic element 15B. In other words, the distance between connector 12 and ultrasonic element 15A is shorter than the distance between connector 12 and ultrasonic element 15B. A power supply circuit 40 is provided near the portion of control unit 13 to which connector 12 is connected. In pulse Doppler systems, the frequency of the ultrasonic signal received by ultrasonic element 15 is easily affected by the power supply circuit 40. For this reason, it is preferable that ultrasonic element 15B, which is located a relatively large distance from connector 12, receive the ultrasonic signal using the pulse Doppler system.

Figure 6 is a diagram showing ultrasonic signals (Doppler signals) detected over time. The horizontal axis of Figure 6 represents frequency. The vertical axis of Figure 6 represents Doppler signal intensity. In the example of Figure 6, multiple Doppler signals s1 to s4 are represented by solid lines, dotted lines, dashed lines, and dashed double-dashed lines, respectively. Doppler signals s1 to s4 are Doppler signals detected for ultrasonic waves reflected by microbubbles flowing at depths d1 to d4 in Figure 5, and are detected at multiple different times t1 to t4, respectively.

The control unit 13 measures the center of gravity positions of the Doppler signals s1 to s4 as Doppler frequencies corresponding to the depths d1 to d4, respectively. The control unit 13 also calculates the flow velocity of the fluid (microbubbles) at each of the depths d1 to d4 based on the measured Doppler frequencies. As mentioned above, the flow velocity of the fluid is calculated by dividing the cross-sectional area by the flow rate V in equation (2).

In this way, the pulse Doppler method can spatially resolve the position of the fluid (microbubbles) and identify the velocity distribution of the fluid. In the examples of Figures 5 and 6, the fluid flow velocity is calculated for each depth d1 to d4, but the number of depths may be determined appropriately depending on the diameter of the pipe P or the processing speed of the control unit 13, etc. The control unit 13 averages the identified fluid velocity distribution and calculates the fluid flow rate by multiplying the averaged velocity distribution by the cross-sectional area of the flow path of the pipe P.

In hybrid mode, the sensor head 10 alternates between transit time differential and pulse Doppler methods. The operation period for transit time differential and pulse Doppler methods is, for example, 150 ms each, but this is not a limitation of the embodiment. The operation period for transit time differential or pulse Doppler methods may be shorter or longer than 150 ms. Furthermore, the operation period for transit time differential and pulse Doppler methods do not have to be the same.

In addition, in this example, ultrasonic element 15B transmits and receives ultrasonic signals using the transit time differential method, and transmits pulsed ultrasonic signals using the pulse Doppler method, measuring the frequency of the ultrasonic waves reflected by the reflector. This configuration allows the number of ultrasonic elements to be minimized in the sensor head 10, which is capable of calculating flow rate using both the transit time differential method and the pulse Doppler method, and has a certain effect on miniaturizing the sensor head 10.

Sensor head 10, which is capable of calculating flow rates using both transit time and pulse Doppler methods, may be configured so that ultrasonic elements used for transit time flow rate calculations and ultrasonic elements used for pulse Doppler flow rate calculations are provided separately. For example, in this embodiment, ultrasonic elements 15A and 15B are used only for transit time flow rate calculations, and ultrasonic elements used for pulse Doppler flow rate calculations are provided separately. In this configuration, the ultrasonic elements used for transit time flow rate calculations are not used for pulse Doppler flow rate calculations, and the ultrasonic elements used for pulse Doppler flow rate calculations are not used for transit time flow rate calculations. Therefore, in the hybrid mode with this configuration, the operating periods using transit time and pulse Doppler methods may overlap, and the sensor is less susceptible to changes in flow rate when the operating periods switch.

Figure 7 shows the flow rate of a fluid calculated using the transit time method and the pulse Doppler method. The horizontal axis of Figure 7 shows the density of microbubbles contained in the fluid relative to the actual flow rate. The vertical axis of Figure 7 shows the relative value of the calculated fluid flow rate. The flow rate measured using the transit time method is shown by a thick solid line, and the flow rate measured using the pulse Doppler method is shown by a thin solid line.

As shown in Figure 7, the transit time method calculates a flow rate of 100% when the microbubble density is relatively low. However, when the microbubble density is relatively high, the flow rate cannot be calculated. On the other hand, the pulse Doppler method calculates a flow rate close to 100% when the microbubble density is relatively high. In particular, when the microbubble density is between value V1 and value V2, which is higher than value V1, the flow rate is calculated with relatively high accuracy.

In hybrid mode, the control unit 13 treats the calculated flow rate as a combination of the flow rate calculated using the transit time differential method and the flow rate calculated using the pulse Doppler method. Therefore, in hybrid mode, the switching signal described above is generated based on the combined flow rate value of the transit time differential method and the pulse Doppler method, and the indicator light 37 lights up or flashes based on the combined flow rate value of the transit time differential method and the pulse Doppler method.

In hybrid mode, the control unit 13 determines the combined ratio of the flow rate calculated using the transit time method and the flow rate calculated using the pulse Doppler method based on the stability of the flow rate calculation. The stability of the flow rate calculation includes, for example, the strength of the detected Doppler signal. In hybrid mode, the control unit 13 may correct the calculated flow rate based on parameters such as the outer diameter of the pipe P or the dynamic viscosity of the fluid.

(3) Ultrasonic Frequency The frequency of the ultrasonic waves transmitted by the ultrasonic element 15B is 4 MHz regardless of whether the transit time method or the pulse Doppler method is used. In particular, in the pulse Doppler method, ultrasonic waves having a frequency of 4 MHz are controlled and transmitted in a pulsed form. Because ultrasonic waves of the same frequency are transmitted in both the transit time method and the pulse Doppler method, it is easy to adopt an ultrasonic element suitable for that frequency, and high signal strength is easily obtained.

This section explains the frequency band that includes the frequency of the ultrasound transmitted by the ultrasonic element 15 in the pulse Doppler method.

The ultrasonic waves transmitted from the ultrasonic element 15 are gradually attenuated as they pass through the components of the sensor head 10, such as the wedge material 14 and acoustic couplant 16, as well as the piping P, before reaching the inside of the piping P. Therefore, the system consisting of the sensor head 10 and the piping P can be considered a filter that attenuates ultrasonic waves. Hereinafter, the filter consisting of the sensor head 10 and the piping P will be referred to as a structural filter. The transmittance of ultrasonic waves through a structural filter varies depending on the frequency of the ultrasonic waves.

Figure 8 shows a typical example of the frequency characteristics of a structural filter. The horizontal axis of Figure 8 represents frequency. The vertical axis of Figure 8 represents ultrasonic transmittance, which is a value when the transmittance of ultrasonic waves at a frequency of 5 MHz is set to 1. As shown in Figure 8, structural filters have high transmittance in relatively low frequency bands, and the ultrasonic transmittance of structural filters decreases as the frequency band increases. On the other hand, the directivity of ultrasonic waves improves as the frequency increases. Therefore, it has been considered preferable to lower the ultrasonic frequency within an acceptable range for reducing directivity.

Here, the inventors conducted various experiments and considerations to select the optimal ultrasonic frequency when a fluid containing microbubbles is flowing through the pipe P, in order to ensure that the sensor head 10 operates appropriately using the pulse Doppler method. As a result, they discovered that lowering the ultrasonic frequency is not always desirable.

Specifically, it was confirmed that microbubbles have a specific resonant frequency, and when irradiated with ultrasound having a frequency close to the resonant frequency, the microbubbles absorb the ultrasound and convert it into heat by repeatedly expanding and contracting. It was also confirmed that the resonant frequency of microbubbles varies depending on the size (diameter) of the microbubbles.

The inventors therefore conducted preliminary experiments to measure the size of microbubbles contained in a fluid. As a result, they found that when the fluid is, for example, a commercially available coolant, the microbubbles contained in the fluid range in size from several μm to several mm, and that the fluid contains a particularly large amount of microbubbles between 10 μm and 50 μm. The resonant frequency band for microbubbles between 10 μm and 50 μm is 50 kHz to 0.4 MHz. Because the frequency band in which microbubbles absorb ultrasound is relatively low, it is preferable that the frequency of the ultrasound transmitted by the ultrasonic element 15 be included in a frequency band higher than the frequency band in which microbubbles absorb ultrasound.

Based on the above findings regarding microbubbles, the inventors evaluated the resonant frequency characteristics of microbubbles contained in a fluid. Figure 9 shows the resonant frequency characteristics of microbubbles contained in a fluid. The horizontal axis of Figure 9 represents frequency. The vertical axis of Figure 9 represents the transmittance of ultrasound per mm of depth, with the transmittance of ultrasound at a frequency of 5 MHz set to 1. As shown in Figure 9, microbubbles contained in a fluid have high transmittance in relatively high frequency bands. In particular, in the frequency band of 1.5 MHz to 5 MHz, the transmittance increases in proportion to the ultrasound frequency. In the frequency band of 5 MHz and above, the transmittance also increases in proportion to the ultrasound frequency, but the change (slope) in transmittance with respect to frequency is smaller.

On the other hand, in the frequency band below 1.5 MHz (part A in Figure 9), transmittance is not proportional to frequency but decreases significantly as frequency decreases. In particular, transmittance is extremely low in the frequency band from 600 kHz to 800 kHz. Therefore, if the ultrasonic frequency is lowered to prevent ultrasonic attenuation in the structural filter, ultrasonic waves will be significantly attenuated in the fluid in the pulse Doppler method. For this reason, it is preferable that the ultrasonic frequency used to calculate the flow rate of a fluid containing microbubbles using the pulse Doppler method be 2 MHz or higher. Furthermore, while the microbubble transmittance increases proportionally to the frequency, the slope of this increase is small in frequency bands higher than 5 MHz. In contrast, the frequency characteristics of a structural filter show that the transmittance decreases as the frequency increases. For this reason, it is preferable that the frequency used to calculate the flow rate of a fluid containing microbubbles using the pulse Doppler method be 5 MHz or lower, as this is likely to improve microbubble transmittance as the frequency increases. Based on these results, it is more preferable to use a predetermined frequency in the frequency band from 2 MHz to 5 MHz as the ultrasonic frequency in the pulse Doppler method.

In this example, the frequency of the ultrasound transmitted in the transit time differential method is 4 MHz, the same as the frequency of the ultrasound transmitted in the pulse Doppler method. However, the frequency of the ultrasound transmitted in the transit time differential method may be different from the frequency of the ultrasound transmitted in the pulse Doppler method. The frequency of the ultrasound transmitted in the transit time differential method may be lower than the frequency of the ultrasound transmitted in the pulse Doppler method. In this case, the attenuation of the ultrasound in the structural filter is reduced. The directionality of the ultrasound is also improved. In particular, the transit time differential method requires that the ultrasound signal travel from one ultrasonic element 15 to the other ultrasonic element 15. Even if an ultrasound signal with a frequency that takes into account the absorption band of microbubbles is used as described above, the ultrasound signal is scattered by the microbubbles, making it difficult for the ultrasound signal to travel from one ultrasonic element 15 to the other ultrasonic element 15. For this reason, it is difficult to calculate the flow rate of a fluid containing a large number of microbubbles using the transit time differential method, regardless of the frequency of the ultrasound signal. Therefore, the frequency of the ultrasound transmitted in the transit time differential method may be set low enough to be within the absorption band of the microbubbles.

In this case, the ultrasonic elements 15 may be driven with different voltages so that ultrasonic waves having different frequencies are transmitted for the transit time differential method and the pulse Doppler method. Alternatively, different ultrasonic elements 15 may be provided so that ultrasonic waves having different frequencies are transmitted for the transit time differential method and the pulse Doppler method.

(4) Flow Rate Correction As described above, the pulse Doppler method can calculate the flow rate with high accuracy even when the density of microbubbles contained in the fluid is relatively high. However, when the density of microbubbles is higher than value V2 in Figure 7, the accuracy of the flow rate calculation decreases. The reason for this is explained below.

Figure 10 shows the Doppler signal detected when the microbubble density is high. The horizontal axis of Figure 10 represents frequency. The vertical axis of Figure 10 represents the intensity of the detected Doppler signal. In the example of Figure 10, consider the Doppler signal detected at time t4. In this case, the Doppler signal s4 is received for ultrasound reflected by microbubbles flowing at depth d4 in Figure 5 (see Figure 6).

However, when the density of microbubbles is high, the amount of ultrasonic attenuation increases, and the intensity of the Doppler signal for ultrasonic waves from microbubbles flowing at depth d4 decreases. Furthermore, the amount of ultrasonic waves reflected from microbubbles flowing at depths shallower than d4 increases. Here, when ultrasonic waves are diffusely reflected by microbubbles flowing at depths shallower than d4, the diffusely reflected ultrasonic waves arrive at the ultrasonic element 15 with a delay compared to when they are directly reflected. A signal for ultrasonic waves that are diffusely reflected in this way and reach the ultrasonic element 15 is called a false signal.

When the diffusely reflected ultrasound arrives at time t4, the same time as the ultrasound reflected by microbubbles flowing at depth d4, a Doppler signal is detected that is a combination of the Doppler signal s4 that should have been detected and the false signal. In the example of Figure 10, the Doppler signal s4 overlaps with the false signals sa-sd, resulting in a Doppler signal s4' that is larger and thicker than the Doppler signal s4. For ease of visualization, the false signals sa-sd are shown as thin dotted lines, thin dashed lines, thick dotted lines, and thick dashed lines, respectively. Furthermore, the Doppler signals s4 and s4' are shown as thin solid lines and thick solid lines, respectively.

In this case, the Doppler frequency corresponding to depth d4 is measured based on Doppler signal s4' rather than Doppler signal s4. If the Doppler frequency measured in this way is used, the calculated average flow velocity of the fluid will be lower than the actual value. Therefore, the calculated flow rate of the fluid will also be lower than the actual value. As a result, the accuracy of the flow rate calculation will decrease.

To prevent this kind of decrease in accuracy, the control unit 13 detects the density of microbubbles contained in the fluid based on a predetermined index. The index for detecting the microbubble density includes, for example, the strength or width of the Doppler signal detected for each depth within the pipe P. In addition, the memory unit of the control unit 13 pre-stores a table showing the relationship between the microbubble density and the amount of flow rate correction. The control unit 13 corrects the calculated flow rate based on the detected microbubble density and the table.

Furthermore, when calculating the Doppler frequency corresponding to each depth d1 to d4, if the amount of shift relative to the frequency of the transmitted ultrasound is greater than one phase cycle, the amount of shift relative to the phase one cycle after the original phase will be calculated, rather than the original phase. This phenomenon is known as cycle skipping, and occurs relatively frequently when the interval between ultrasound phases is short, as in this embodiment, i.e., when the ultrasound frequency is high. When a cycle skip occurs, a Doppler frequency that is lower than the original value will be calculated, and therefore a flow rate that is lower than the original value will be calculated.

The control unit 13 then determines whether a cycle skip has occurred. Specifically, the control unit 13 determines that a cycle skip has occurred when the shift amount calculated for a certain depth deviates by a predetermined value or more from the value previously calculated for that depth. Note that in the hybrid method, the control unit 13 may also determine that a cycle skip has occurred in the calculation of the Doppler frequency corresponding to any depth when the calculated flow rate deviates by a predetermined value or more from the value obtained using the transit time method.

If a cycle skip is determined to have occurred in the calculation of the shift amount at any of depths d1 to d4, the control unit 13 treats the flow velocity at the depth where the cycle skip is determined to have occurred as a value calculated using a calculation with low stability when averaging the velocity distribution. Specifically, the velocity distribution is averaged based only on flow velocities at depths where a cycle skip is not determined to have occurred. Note that the velocity distribution may be averaged based on a flow velocity corrected to a value based on the shift amount when no cycle skip occurs, instead of the flow velocity at the depth where the cycle skip is determined to have occurred. This correction may be performed by adding a correction value determined to correspond to the ultrasonic frequency, or by multiplying by a correction factor determined to correspond to the ultrasonic frequency. Furthermore, the average flow velocity in the pipe P may be calculated based on the flow velocity calculated using the value calculated in the previous Doppler frequency calculation, instead of the flow velocity at the depth where the cycle skip is determined to have occurred. Furthermore, corrections to calculations determined to be with low stability are not limited to flow rates calculated using the pulse Doppler method. For example, in hybrid mode, the control unit 13 may correct the combination ratio of the flow rate calculated by the pulse Doppler method when combining the flow rate calculated by the pulse Doppler method and the flow rate calculated by the transit time method, taking into account the influence of cycle skips.

(5) Flow Rate Calculation Process Fig. 11 is a block diagram showing the configuration of the control unit 13 in Fig. 2. As shown in Fig. 11, the control unit 13 includes a measurement unit 1, a calculation unit 2, a determination unit 3, a detection unit 4, and a determination unit 5 as functional units, as well as a storage unit 6. A flow rate calculation program is stored in the storage unit 6. Parameters such as the velocity C', the angle of incidence θ', the angle of incidence θ, and the Blasius coefficient are also stored in advance in the storage unit 6. The CPU of the control unit 13 executes the flow rate calculation program stored in the storage unit 6, thereby realizing the functional units of the control unit 13. Some or all of the functional units of the control unit 13 may be realized by hardware such as electronic circuits.

The measurement unit 1 controls the operation of the ultrasonic elements 15A and 15B and acquires measurement results from the ultrasonic elements 15A and 15B. The calculation unit 2 calculates the flow rate through the pipe P based on the measurement results acquired by the measurement unit 1, the parameters stored in the memory unit 6, and the parameters set in the display unit 30. The calculation unit 2 also performs a predetermined correction on the calculated flow rate in the pulse Doppler method.

In hybrid mode, the determination unit 3 determines the combined ratio of the flow rate calculated using the transit time method and the flow rate calculated using the pulse Doppler method. The detection unit 4 detects the density of microbubbles contained in the fluid at a set cycle using the pulse Doppler method. The judgment unit 5 determines whether a cycle skip has occurred using the pulse Doppler method.

Figures 12 and 13 are flowcharts showing an example of an algorithm for the flow rate calculation process executed in hybrid mode by the control unit 13 in Figure 11. Below, the flow rate calculation process in hybrid mode will be explained using the control unit 13 in Figure 11 and the flowcharts in Figures 12 and 13.

First, the measurement unit 1 transmits an ultrasonic signal to the ultrasonic element 15A and controls the ultrasonic elements 15A and 15B so that the ultrasonic signal is received by the ultrasonic element 15B (step S1). Next, the measurement unit 1 measures the propagation time of the ultrasonic signal from the ultrasonic element 15A to the ultrasonic element 15B in step S1 (step S2).

Then, the measurement unit 1 transmits an ultrasonic signal to the ultrasonic element 15B and controls the ultrasonic elements 15A and 15B so that the ultrasonic signal is received by the ultrasonic element 15A (step S3). Next, the measurement unit 1 measures the propagation time of the ultrasonic signal from the ultrasonic element 15B to the ultrasonic element 15A in step S3 (step S4). Either steps S1 and S2 or steps S3 and S4 may be executed first.

Next, calculation unit 2 calculates the propagation time difference based on the propagation times measured in steps S2 and S4 (step S5). Calculation unit 2 also calculates the flow rate of the fluid flowing through pipe P based on the propagation time difference calculated in step S5 and equation (1) (step S6). Steps S1 to S6 correspond to processing in the propagation time difference method.

After step S6, the measurement unit 1 controls the ultrasonic element 15B to sequentially transmit and receive pulsed ultrasonic signals (step S7). Next, the measurement unit 1 measures the Doppler frequency of the ultrasonic signal received in step S7 (step S8). Furthermore, the calculation unit 2 calculates the flow velocity of the fluid flowing through the pipe P at each depth d1 to d4 based on the Doppler frequency measured in step S8 (step S9).

The determination unit 5 determines whether a cycle skip has occurred in the frequency shift corresponding to each depth d1 to d4 (step S10). If it determines that a cycle skip has not occurred, the process proceeds to step S12. If it determines that a cycle skip has occurred, the depth at which the cycle skip occurred is identified (step S11). After the depth at which the cycle skip occurred has been identified, the process proceeds to step S12. In step S12, the flow velocity distribution is averaged based on the flow velocity at each depth d1 to d4, and the flow rate is calculated based on the averaged flow velocity. That is, in steps S9 and S12, the flow rate value corresponding to equation (2) is calculated.

Here, the detection unit 4 detects the density of microbubbles contained in the fluid (step S13). Step S13 may be performed in parallel with steps S7 to S9, or may be performed before step S10. The calculation unit 2 corrects the flow rate calculated in step S12 based on the microbubble density detected in step S13 and a predetermined table (step S14). Steps S7 to S14 correspond to processing in the pulse Doppler method. In hybrid mode, either steps S1 to S6 or steps S7 to S14 may be performed first.

In this embodiment, the determination of whether a cycle skip has occurred, which is performed in step S10, and the detection of the density of microbubbles contained in the fluid, which is performed in step S13, are both processes for further improving the accuracy of the flow rate calculated using the pulse Doppler method. Therefore, the processes in steps S10 and S13 are not limited to the processing order shown in Figure 13, as long as they can be reflected in the flow rate calculated using the pulse Doppler method.

After step S14, the determination unit 3 determines the combination ratio of the flow rate calculated in step S6 and the flow rate calculated in step S9 based on the stability of the flow rate calculation, such as the intensity of the Doppler signal detected in step S7 (step S15). Next, the calculation unit 2 calculates the flow rate of the fluid flowing through the pipe P by combining the flow rate calculated in step S6 and the flow rate corrected in step S14 at the combination ratio determined in step S15 (step S16). The calculation unit 2 then outputs the flow rate calculated in step S16 to the control unit 33 of the display unit 30 (step S17), and the process returns to step S1. The control unit 33 of the display unit 30 compares the flow rate output by the algorithm shown in Figures 12 and 13 with a predetermined threshold value and generates a switching signal.

In the transit time difference mode, steps S1 to S6 are executed, and then step S17 is executed without executing steps S7 to S16. In this case, in step S17, the calculation unit 2 outputs the flow rate calculated in step S6 to the display 30, and the process returns to step S1. That is, the control unit 33 of the display 30 compares the flow rate calculated in step S6 with a predetermined threshold value. On the other hand, in the pulse Doppler mode, steps S1 to S6, S15, and S16 are not executed, and steps S7 to S14 are executed, and then step S17 is executed. In this case, in step S17, the calculation unit 2 outputs the flow rate corrected in step S14 to the display 30, and the process returns to step S7. That is, the control unit 33 of the display 30 compares the flow rate corrected in step S14 with a predetermined threshold value.

(6) Effects In the ultrasonic flow sensor 100 according to this embodiment, the pulsed ultrasonic signal transmitted by the ultrasonic element 15B using the pulse Doppler method has a predetermined frequency (2 MHz to 5 MHz in this example) higher than the absorption band of microbubbles contained in the fluid. In this case, attenuation of the ultrasonic signal due to microbubbles is reduced. As a result, the flow rate of the fluid can be measured even if the fluid flowing through the pipe P contains microbubbles.

In addition, in the pulse Doppler method, the velocity distribution of the fluid flowing within the pipe P is determined based on Doppler signals detected at different times, and the flow rate of the fluid is calculated based on the average value of the determined velocity distribution of the fluid. In this case, the flow rate of the fluid can be measured more accurately. In this example, the ultrasonic element 15B is composed of a composite element, so the reverberation time of the transmitted ultrasound is short. As a result, the velocity distribution of the fluid flowing within the pipe P can be determined more accurately.

In the transit time differential method, the fluid flow rate is calculated based on the difference in transit time of the ultrasonic signal between ultrasonic elements 15A and 15B. Therefore, the fluid flow rate can be accurately measured even when the density of microbubbles contained in the fluid is low. In hybrid mode, the combination ratio of the flow rate calculated using the transit time differential method and the flow rate calculated using the pulse Doppler method is determined based on the stability of the flow rate calculation. The fluid flow rate is calculated by combining the flow rate calculated using the transit time differential method and the flow rate calculated using the pulse Doppler method at the determined combination ratio. In this case, it becomes easy to accurately measure the fluid flow rate regardless of the density of microbubbles contained in the fluid.

In addition, with the pulse Doppler method, the density of microbubbles contained in the fluid is detected, and the calculated flow rate is corrected based on the detected microbubble density. With this configuration, the flow rate of the fluid can be accurately measured even if the density of microbubbles contained in the fluid is higher than value V2 in Figure 7.

Furthermore, with the pulse Doppler method, it is determined whether a cycle skip has occurred when calculating the flow rate, and if it is determined that a cycle skip has occurred, the calculated flow rate is corrected to the value that would be obtained if no cycle skip had occurred. With this configuration, the flow rate of the fluid can be accurately measured even if a cycle skip has occurred when calculating the flow rate.

The sensor head 10 and the display 30 can be connected using either the first or second connection method. With the first connection method, the housing 31 of the display 30 is attached to the housing 11 of the sensor head 10, thereby directly connecting the connector 32 and the connector 12. With the second connection method, the connector 32 and the connector 12 are connected via a cable. With the first connection method, the user can easily recognize the displayed information by visually checking the display 30 attached to the sensor head 10. With the second connection method, the user can easily recognize the displayed information by visually checking the display 30 attached in any position.

(7) Other Embodiments (a) In the above embodiment, the sensor head 10 calculates the flow rate using both the transit time method and the pulse Doppler method, but the embodiment is not limited to this. The sensor head 10 may be configured to operate in pulse Doppler mode. Therefore, the sensor head 10 may be configured to be able to calculate the flow rate using the pulse Doppler method, and may not calculate the flow rate using the transit time method. In this case, the sensor head 10 may be provided with an ultrasonic element that can calculate the flow rate using the pulse Doppler method, and may not include the ultrasonic element 15A in this example.

(b) In the above embodiment, the flow rate of the fluid in the pulse Doppler method is calculated based on the average value of the velocity distribution of the fluid flowing through the pipe P, but the embodiment is not limited to this. The flow rate of the fluid in the pulse Doppler method may be calculated based on a representative value other than the average value, such as a median value, determined by the velocity distribution of the fluid flowing through the pipe P.

(c) In the above embodiment, ultrasonic element 15B receives ultrasonic waves reflected by microbubbles, but the embodiment is not limited to this. An ultrasonic element other than ultrasonic element 15B may receive ultrasonic waves reflected by microbubbles. Alternatively, an ultrasonic element for receiving ultrasonic waves reflected by microbubbles may be provided separately from ultrasonic flow sensor 100.

(d) In the above embodiment, the ultrasonic element 15A is provided as part of the ultrasonic flow sensor 100, but the embodiment is not limited to this. The ultrasonic element 15A may be provided separately from the ultrasonic flow sensor 100, as long as ultrasonic waves can be transmitted and received between the ultrasonic element 15A and the ultrasonic element 15B through the fluid flowing in the pipe P.

(e) In the above embodiment, ultrasonic element 15A and ultrasonic element 15B are arranged above pipe P so as to be aligned in the direction in which pipe P extends, but the embodiment is not limited to this. Ultrasonic element 15A and ultrasonic element 15B may also be arranged so as to face each other across pipe P. In other words, one ultrasonic element may be arranged above pipe P and the other ultrasonic element may be arranged below pipe P.

(8) Correspondence between each component of the claims and each part of the embodiment Examples of correspondence between each component of the claims and each part of the embodiment are described below, but the present invention is not limited to the following examples. Various other elements having the configuration or function described in the claims can also be used as each component of the claims.

In the above embodiment, the pipe P is an example of a pipe, the ultrasonic flow sensor 100 is an example of an ultrasonic flow sensor, and the ultrasonic elements 15B and 15A are examples of the first and second ultrasonic elements, respectively. The calculation unit 2 is an example of a calculation unit, the determination unit 3 is an example of a determination unit, the detection unit 4 is an example of a detection unit, and the judgment unit 5 is an example of a judgment unit.

The housing 11 is an example of a sensor head housing, the sensor head 10 is an example of a sensor head, the display 30 is an example of a display, the connector 12 is an example of a sensor head connector, and the connector 32 is an example of a display connector. The display 36 is an example of a display, the control unit 33 is an example of a display control unit, the housing 31 is an example of a display housing, and the indicator light 18 is an example of a sensor head indicator light.

1...Measuring unit, 2...Calculating unit, 3...Determining unit, 4...Detecting unit, 5...Determining unit, 6...Memory unit, 10...Sensor head, 11, 31...Housing unit, 12, 32...Connector, 13, 33...Control unit, 14, 14A, 14B...Wedge material, 14a...Element coupling surface, 14b...Pipe coupling surface, 15, 15A, 15B...Ultrasonic element, 16...Acoustic couplant, 17...Ultrasonic shielding plate, 20...Clamping unit, 21...Upper clamping member, 22...Lower clamping member, 23...Clamp fixing screw, 30...Display, 34...Memory element, 35...Operating unit, 36...Display unit, 37...Indicator light, 38, 39...Connection port, 40...Power supply circuit, 100...Ultrasonic flow sensor, 101...Sensor fixing screw, P...Pipe, s1-s4, s4'...Doppler signals, sa-sd...False signals

Claims (9)

An ultrasonic flow sensor for measuring the flow rate of a fluid containing microbubbles flowing in a pipe,
a first ultrasonic element that transmits a pulsed first ultrasonic signal, which is an ultrasonic wave having a predetermined frequency included in a frequency band of 2 MHz to 5 MHz, which is higher than the absorption band of the microbubbles, to the fluid;
a second ultrasonic element that receives a second ultrasonic signal having the predetermined frequency transmitted from the first ultrasonic element to the fluid and transmits a third ultrasonic signal received by the first ultrasonic element to the fluid;
an ultrasonic flow sensor comprising: a calculation unit that calculates , as a first method of flow rate calculation , a first flow rate of the fluid based on the difference between the predetermined frequency and the frequency of an ultrasonic signal reflected by the microbubbles; and, as a second method of flow rate calculation, calculates a second flow rate of the fluid based on a propagation time difference between the second ultrasonic signal and the third ultrasonic signal between the first ultrasonic element and the second ultrasonic element; and outputs a calculated flow rate based on at least one of the first flow rate and the second flow rate, while dealing with microbubbles having a density that allows flow rate calculation by both the first method and the second method and also dealing with microbubbles having a density that exceeds a range of densities that allows flow rate calculation by both the first method and the second method but allows flow rate calculation by only one of the first method and the second method . The calculation unit
calculating a velocity of the fluid at a first depth in the pipe based on a difference between the predetermined frequency and a frequency of an ultrasonic signal reflected by the microbubbles at a first time point after a predetermined time has elapsed since the ultrasonic signal was transmitted by the first ultrasonic element;
calculating a velocity of the fluid at a second depth in the pipe based on a difference between the predetermined frequency and a frequency of an ultrasonic signal reflected by the microbubbles at a second time point different from the first time point after the ultrasonic signal is transmitted by the first ultrasonic element;
2. The ultrasonic flow sensor according to claim 1, wherein the first flow rate is calculated based on a representative value of a velocity distribution of the fluid determined based on the velocity of the fluid at the first depth and the velocity of the fluid at the second depth. 3. The ultrasonic flow sensor according to claim 1, wherein the first ultrasonic element is a composite element. a determination unit that determines a combination ratio of the first flow rate and the second flow rate based on the stability of the flow rate calculation,
4. The ultrasonic flow sensor according to claim 1, wherein the calculation unit calculates a third flow rate of the fluid by combining the first flow rate and the second flow rate at the combination ratio determined by the determination unit. Further, a detection unit is provided to detect the density of the microbubbles contained in the fluid,
The ultrasonic flow sensor according to claim 1 , wherein the calculation unit corrects the first flow rate calculated based on the density of the microbubbles detected by the detection unit. a determination unit that determines whether a cycle skip has occurred when calculating the first flow rate,
6. The ultrasonic flow sensor according to claim 1, wherein the calculation unit corrects the calculated first flow rate when the determination unit determines that a cycle skip has occurred. The ultrasonic flow sensor according to claim 1 , further comprising a sensor head having the first ultrasonic element and a sensor head housing that houses the first ultrasonic element. further comprising a display in communication with the sensor head;
the sensor head has a sensor head connector;
The display includes:
a display connector connected to the sensor head connector;
a display unit that displays display information based on the calculated flow rate transmitted from the sensor head;
a display control unit that controls the display unit;
a display housing that houses the display control unit,
8. The ultrasonic flow sensor according to claim 7, wherein the sensor head connector and the display connector are connected by a first connection method in which the display housing is attached to the sensor head housing to connect them directly, and a second connection method in which the display housing is connected via a cable . the sensor head is provided with a sensor head indicator light;
the indicator control unit generates a switching signal indicating either that the flow rate calculated by the calculation unit is equal to or greater than a predetermined threshold value or that the flow rate is smaller than the predetermined threshold value;
9. The ultrasonic flow sensor according to claim 8 , wherein the sensor head indicator light displays a result of a comparison between the calculated flow rate and the predetermined threshold value.